1. Field of the Invention
The present invention relates to an optical recording medium that records and plays back information in response to blue laser (of 300 to 500 nm in wavelength) and has a recording layer containing organic dye.
2. Description of the Related Art
Optical discs and other optical information recording media are becoming popular as information recording media. Some of these optical information recording media have a reflection layer and recording layer formed, on top of each other, on a light transmission resin substrate of 1.2 mm in thickness and 120 mm or 80 mm in diameter, such as recordable compact discs (CD-Rs). These optical recording media record data by forming recording marks in their recording layer in response to irradiation of a recording laser beam. To achieve even higher information recording density, other methods have been devised whereby the laser wavelength is shortened and an objective lens of large NA (Numerical Aperture) is used, which are embodied in recordable digital versatile discs (DVD±Rs) and other similar optical information recording media. DVD±Rs are structured in such a way that two light-transmission resin substrates of 0.6 mm thickness are bonded together, with a reflection layer and recording layer sandwiched between these substrates, in order to increase the allowable tilt of the disc that must be angled due to its shorter wavelength and higher NA. In recent years, higher information recording density is further required to record high-definition image data. To meet this requirement, recordable Blu-ray discs (BD-R: Blu-ray Disc is a registered trademark) and other similar optical information recording media have emerged, which are structured in such a way that a reflection layer and recording layer are formed on the light-entering side of a resin substrate of 1.1 mm in thickness, with a light-transmission cover layer (light transmission layer) provided on the side where the reflection layer and recording layer are formed.
With these optical information recording media, serial number, lot number, and other management information are bar-coded and recorded on each optical disc so that whether or not it is a genuine optical disc manufactured or sold by the specified manufacturer or distributor can be identified using this management information. To be specific, a burst cutting area (hereinafter referred to as “BCA”) is provided on the innermost periphery side of the area over which the recording layer of the optical disc is formed, and a barcode-shaped mark (hereinafter referred to as “BCA Mark”) is recorded in the recording layer inside this area, to allow for identification of the optical disc by reading the BCA mark using an optical head equipped on any drive designed to play back optical discs. Even a part of the BCA mark is used to prevent illegal copying of information. Accordingly, the BCA mark is an important and essential aspect of optical discs.
To improve the BCA cutting quality on optical discs, ingenious ways have been devised to improve the quality of the BCA when it is cut, including the (1) technology whereby, in a medium having two or more recording layers, the heat conductivity of the material constituting other layer(s) is adjusted to at least 1.5 times the heat conductivity of the material constituting the information recording layer where the BCA mark is to be processed, in order to facilitate heat diffusion in the information recording layer and prevent processing marks from remaining in the layer that should not be damaged (Japanese Patent Laid-open No. 2005-196940) and (2) technology whereby a reflective film is provided using Ag as the main constituent and Gd as a trace additive, to allow more laser marking beam to be absorbed and thereby improve the recording characteristics of the BCA (Japanese Patent Laid-open Nos. 2007-335061 and 2008-117470), etc. It should be noted that all of these technologies are designed to improve the BCA cutting quality on optical disc media whose recording layer is made of inorganic material.